Regulation of hepatic fat and glucose oxidation in rats with lipid-induced hepatic insulin resistance

Abstract

Pyruvate dehydrogenase plays a critical role in the regulation of hepatic glucose and fatty acid oxidation; however, surprisingly little is known about its regulation in vivo. In this study we examined the individual effects of insulin and substrate availability on the regulation of pyruvate dehydrogenase flux (V(PDH) ) to tricarboxylic acid flux (V(TCA) ) in livers of awake rats with lipid-induced hepatic insulin resistance. V(PDH) /V(TCA) flux was estimated from the [4-(13) C]glutamate/[3-(13) C]alanine enrichments in liver extracts and assessed under conditions of fasting and during a hyperinsulinemic-euglycemic clamp, whereas the effects of increased plasma glucose concentration on V(PDH) /V(TCA) flux was assessed during a hyperglycemic clamp in conjunction with infusions of somatostatin and insulin to maintain basal concentrations of insulin. The effects of increases in both glucose and insulin on V(PDH) /V(TCA) were examined during a hyperinsulinemic-hyperglycemic clamp. The effects of chronic lipid-induced hepatic insulin resistance on this flux were also examined by performing these measurements in rats fed a high-fat diet for 3 weeks. Using this approach we found that fasting V(PDH) /V(TCA) was reduced by 95% in rats with hepatic insulin resistance (from 17.2 ± 1.5% to 1.3 ± 0.7%, P < 0.00001). Surprisingly, neither hyperinsulinemia per se or hyperglycemia per se were sufficient to increase V(PDH) /V(TCA) flux. Only under conditions of combined hyperglycemia and hyperinsulinemia did V(PDH) /V(TCA) flux increase (44.6 ± 3.2%, P < 0.0001 versus basal) in low-fat fed animals but not in rats with chronic lipid-induced hepatic insulin resistance.

Conclusion: These studies demonstrate that the combination of both hyperinsulinemia and hyperglycemia are required to increase V(PDH) /V(TCA) flux in vivo and that this flux is severely diminished in rats with chronic lipid-induced hepatic insulin resistance.

Figure 1

Schematic of metabolite labeling pattern following [1-¹³C]glucose infusion. V_PDH and V_TCA represent the fluxes through pyruvate dehydrogenase (PDH) and tricarboxylic acid cycle (TCA), respectively.

Figure 2

Effect of insulin and substrate on the hepatic contribution of pyruvate dehydrogenase flux (V_PDH) to total TCA cycle flux (V_TCA). (A) V_PDH/V_TCA during fasting, a hyperinsulinemic euglycemic clamp and a hyperinsulinemic hyperglycemic clamp performed in low-fat and high-fat fed animals; (B) Effect of insulin per se and insulin plus glucose on the concentrations of glycogen; (C) Plasma fatty acid concentrations during a hyperinsulinemic euglycemic clamp in low-fat and high-fat fed animals and during a concomitant infusion of Liposyn (* P<0.01, ** P<0.001 vs. Fasting); (D) Effect of sustained concentrations of plasma fatty acids on hepatic V_PDH/V_TCA in low-fat fed animals.

Figure 3

Effect of high-fat diet on the regulators of PDH. (A) Protein levels of pyruvate dehydrogenase kinase (PDK2), PDK4 and peroxisome proliferator-activated receptor α (PPARα); (C) Phosphorylation levels of Ser232 of PDH complex.